Question regarding the "Find Weakness" proc for Shadow (Infiltration Spec)

I have a question regarding the Infiltration tree. How do I know when Infiltration Tactics (Find Weakness) procs? I tried doing some searching on various sites but could not find the answer.

I have fought countless enemies and keep an eye on their debuffs (is that the right term?) and never saw anything to indicate that they are afflicted with "Find Weakness".

It says it procs off of direct damage. What exactly causes "direct damage"?

Thanks!